The production of hydrogen and ethanol as well as other fermentation products from biomass, particularly plant biomass, is becoming an increasingly attractive option for alternative fuel production as prices of fossil fuels and petroleum increase. As fossil fuels become depleted alternative energy sources will become a crucial area of research both in industry and in academia.
In particular hydrogen is recognized as a clean and recyclable energy carrier and there is a prominent thrust in research initiatives focusing on the sufficient, efficient, profitable and “green” production of hydrogen gas. It is believed that hydrogen gas as an alternative energy carrier is indeed one of the more promising alternatives to be considered and exploited in the future. The use of biomass in the production of hydrogen gas provides for a “green” solution for hydrogen production which is hoped will be optimized and developed to provide a means for providing an economical and profitable supply of hydrogen gas. Furthermore, biological production of hydrogen from organic wastes as well as from other recyclable resources is considered preferable to the production of hydrogen from food crops for, while the hydrogen yield of food crops such as maize and wheat is relatively high, there is a global food shortage which is in danger of becoming exacerbated by the use of food crops in biological hydrogen producing reactors.
WO2009/034439, having the same inventor as the current application and fully incorporated hereto by reference, describes a bioreactor system for the rapid screening, selection and isolation of biofilm, floc and granule forming thermophilic bacteria or bacteria consortia that generate high levels of hydrogen from plant biomass including the soluble hydrolysates derived from the hydrolysis of cellulosic materials and particularly of cellulosic materials such as sugar cane waste and effluent that been subjected to only minimum pretreatment such as milling and wet heating.
Recent developments in utilizing fermentation processes in the production of hydrogen have pointed to advantages in using mesophiles and thermophiles in the process. Thermophiles, including extreme thermophiles, have many advantages as agents for the generation of biohydrogen from cellulose and from soluble hydrolysate derived from cellulose hydrolysis. Perhaps their main advantage is that high temperatures exclude microbial contamination from a bioreactor system. High temperatures also shift the equilibrium constant for the hydrogen generating reactions in the forward direction thereby increasing the hydrogen yield (HY). Most thermophiles and extreme thermophiles are, however, difficult to culture and maintain as pure cultures although it has been found that the hydrolysis of cellulosic materials and the generation of hydrogen from the products of this hydrolysis becomes increasingly favourable under the action of a mixed consortium of bacteria that includes anaerobic cellulolytic bacterial species.
The recent flood of reviews on biohydrogen production is an indication that current advances in biohydrogen generation technology has now entered or even gone beyond the mature phase of development (Das 2007; Davila-Vazquez et al 2007; Hallenbeck 2009; Hallenbeck and Gosh 2009; Hawkes et al 2007; Liu et al 2008; Tsyganov 2007; Valdez-Vazquez et al 2009). Attempts to improve both the productivity (HP) and yield (HY) of biohydrogen generation in dark anaerobic processes appears to have now also reached the point of diminishing returns (Rittmann 2008). Under most bioreactor design and operation conditions the maximum possible H2 yield in the anaerobic oxidation of glucose to acetate has generally been observed not to exceed 4 mol H2/mol glucose. In this reaction, of the 24 electron equivalents (e− eq) of glucose, 8 e− eq end up in H2 and the remaining 16 e− eq end up in acetate. In dark fermentation the hydrogen yield (HY) appears to be “stuck” at 4 mol H2/mol glucose (Rittmann 2008). Theoretically acetate could be further oxidized under anaerobic conditions to yield 4H2 and 2CO2 in the absence of methanogens if the partial pressure of H2 in the bioreactor can be reduced. Whether or not a practically viable anaerobic single or multi-stage bioprocess could be engineered that would facilitate the complete oxidation of glucose to 12H2 remains an interesting, but controversial consideration (Hallenbeck 2009; Hallenbeck and Gosh 2009). It remains the general scientific consensus that formidable hurdles need to be overcome before the complete oxidation of glucose to hydrogen at high rates in a multiple stage process can be realized in practice (Hallenbeck and Gosh 2009).
The theoretical maximum value for hydrogen yield (HY) is 4 mol H2/mol glucose. With respect to evaluating bioreactor performance, the critical threshold for the hydrogen yield (HY) can, for practical purposes, be set at 75% of the theoretical maximum, therefore 3 mol H2/mol glucose.
It is of crucial importance to note that in practice hydrogen yield (HY) values equal to or exceeding 3 mol H2/mol glucose are usually only attained in situations where the volumetric hydrogen productivity (HP) is several orders of magnitude below the critical limit of 120 mmol H2/(L·h) (Levin et al 2004).
Conditions that favour high hydrogen yields (HY) can be summarized as follows: thermophilic temperatures, low substrate loading rates, low dilution rates (low hydraulic retention times), low hydrogen partial pressures and low bacterial biomass densities. In addition, H2 gas stripping by sparging with N2 is usually a necessary precondition for the achievement of hydrogen yields (HY) equal to or greater than 3 mol H2/mol glucose. However under these conditions the hydrogen productivity (HP) is several orders of magnitude below the critical threshold of 120 mmol H2/(L·h).
In all the instances where high hydrogen productivities (HP) have been achieved, the following bioreactor operational conditions have prevailed: high substrate loading rates, high dilution rates (high hydraulic rates of retention), and high bacterial biomass densities. Operational conditions that favour high hydrogen productivities (HP) also promote the maintenance of high hydrogen partial pressures within the bioreactor environment. High hydrogen partial pressures within the bioreactor environment do not favour the simultaneous attainment of hydrogen yields (HY) equal to or greater than 3 mol H2/mol glucose.
In general the conditions promoting high hydrogen productivities (HP) do not simultaneously favour the achievement of high hydrogen yields (HY). Recently published surveys show that less than 5% of all reported HY values from a wide diversity of experiments were equal to or greater than 3.0 mol H2/mol glucose (Chong et al 2009; Das 2009; Davila-Vazquez et al 2007; Wang and Wan 2009).
Accordingly, there is a need for a bioreactor system that can utilize a mixed anaerobic bacterial consortium in order to concomitantly produce high HPs and high HYs.